1. Field of the Invention
The present invention relates to a wafer thickness measuring apparatus and a method thereof, and in more details, to a wafer thickness measuring apparatus and a method thereof, applying measurement by means of an optical heterodyne interference thereto, for performing the measurement in the distance between relative positions (i.e., a length measurement), being able to perform absolute length measurement exceeding the wave-length of a light thereof, thereby enabling the measurement with an accuracy by means of the optical heterodyne interference.
2. Description of Prior Art
As one of items to be measured when conducting measurement in the flatness of a wafer, a measurement in thickens of the wafer is listed up. In recent years, the thickness of the wafer comes to be thinner gradually, and therefore, at present, there is required a reproducibility of measurement of length being equal or less than xc2x10.5 xcexcm as the accuracy for it.
The flatness measurement of the wafers, in general, is performed by means of an electrostatic sensor, so as to measure a condition of unevenness or non-uniformity on the surface of the wafer, therefore it is not required to measure such the long distance like the thickness of the wafer.
In a relation that the thickness of the wafer is also included in the items to be performed when conducting the flatness measurement of the wafer, there can be considered that the electrostatic capacity sensor for the measurement on the flatness is also used for the purpose of the measurement on the thickness of the wafer. However, the thickness of the wafer is about equal or greater than 700 xcexcm, exceeding the range of measurement of the flatness measurement by means of the electrostatic capacity sensor, therefore it is impossible to measure it with high accuracy. However, if an other sensor for measurement is provided therewith, there are brought about problems that the apparatus comes to be large-scaled, and that a manufacturing cost thereof rises up.
Also, for the measurement of the thickness by means of the electrostatic capacity sensor or the like, in general, there must be provided a measurement device for measuring a distance from a reference position facing to a front surface side up to the front surface of the wafer and a measurement device for measuring a distance from a reference position facing to a reverse surface side up to the reverse surface of the wafer, and the reference positions facing to the front surface side and the reverse surface side are fixed onto a frame of a measuring apparatus. Then, an error is caused in the reference positions due to expansion and contraction of the frame due to changes of the temperature thereof, or due to secular distortion thereof, therefore it is difficult to keep the measurement of the high accuracy.
An object according to the present invention, for dissolving such the problems of the convention arts as mentioned in the above, is to provide a wafer thickness measuring apparatus and a method thereof by using a measurement instrument of an optical heterodyne interferometer, thereby being able to measure the thickness of the wafer, exceeding the wave-length of the light thereof, with high accuracy.
According to the present invention, for achieving the above-mentioned object, there is provide a wafer thickness measuring apparatus, comprising:
a sample piece being positioned corresponding to height of a wafer and inclined at a predetermined angle substantially corresponding to an inclination angle due to bend of said wafer, and having a constant thickness being already known and equivalent to that of said wafer;
a first detector for generating a detection signal depending upon a distance from a predetermined reference position facing to a front surface side of said wafer up to a position on the front surface of said wafer corresponding to a measurement point of an optical heterodyne interferometer;
a second detector for generating a detection signal depending upon a distance from a predetermined reference position facing to a reverse surface side of said wafer to a position on the reverse surface of said wafer corresponding to a measurement point of said optical heterodyne interferometer;
a data sampling/memorizing means for measuring fluctuations at a large number of the measurement points on the front surface or the reverse surface of said sample piece by means of said optical heterodyne interferometer, and then for memorizing detection values based upon said detection signals of said first and second detectors, which are obtained at said measurement points, depending upon the fluctuation amounts at the respective measurement points; and
means for obtaining the respective detection values upon the detection signals of said first and second detectors at arbitrary measurement points on said wafer, a front surface and a reverse surface of which lie within a range of height between the front surface and the reverse surface of said sample piece, and for obtaining said fluctuation amounts corresponding to said detection values which are measured by said optical heterodyne interferometer and memorized in said data sampling/memorizing means from the respective detection values, respectively, thereby calculating out thickness of said wafer upon basis of the respective fluctuation amounts obtained and the thickness of said sample piece.
Further, according to the present invention, for achieving the object mentioned above, there is provided a wafer thickness measuring apparatus for detecting a thickness of a wafer to be measured, as well as a flatness on a surface thereof, comprising:
an absolute distance detecting means for detecting an absolute distance from a reference position facing a front surface side of said wafer to the front surface of said wafer;
a relative fluctuation detecting means for detecting a relative fluctuation upon the front surface of said wafer;
means for memorizing a plurality of detected values from said absolute distance measuring means and a plurality of detected values from said relative fluctuation detecting means, by making correspondence therebetween; and
a data processing means for calculating the thickness of said wafer, upon basis of the detected values from said absolute distance detecting means and said relative fluctuation detecting means, and the memorized values thereof in said memorizing means, wherein said apparatus further comprises a sample piece being positioned and inclined at a predetermined angle corresponding to an inclination angle due to bend caused in a portion of said wafer when being mounted on said apparatus, wherein said data processing means memorizes the detected values from said absolute distance detecting means and said relative fluctuation detecting means at a plurality of measurement points upon the surface of said sample piece by making correspondence therebetween, and when calculating the thickness of said wafer, said data processing means obtains the relative fluctuation value corresponding to the measurement point upon basis of the detected value of said absolute distance detecting means, thereby to calculate the thickness of said wafer upon basis of comparison between the detected value of said relative fluctuation detecting means and the obtained value of the relative fluctuation.
And further, according to the present invention, there is also provided a wafer thickness measuring method for detecting a thickness of a wafer to be measured, comprising the following steps:
detecting a sample piece being positioned and inclined at a predetermined angle corresponding to an inclination angle due to bend caused in a portion of said wafer to be measured when being mounted on said apparatus, in advance, on an absolute distance from a reference position facing a front surface side of said sample piece to the front surface thereof and an fluctuation upon the surface of said sample piece;
memorizing the detected distance value and the relative fluctuation by making correspondence therebetween;
detecting the absolute distance from said reference position facing a front surface side to the front surface of said wafer to be measured, which is mounted on a measuring apparatus, and the relative fluctuation upon the surface of said wafer; and
picking up the relative fluctuation memorized in advance, by making correspondence with the detected absolute distance, and compare the relative fluctuation picked up with the relative fluctuation detected, thereby to calculate the thickness of said wafer.
In this manner, the detection values are obtained from the sample piece (for example, a wafer chip) at a plurality of measurement points thereon by the first and second detectors for absolute measurement of distances, and at the same time, the measurement values of the fluctuations, for relative measurement of distances, are obtained by the optical heterodyne interferometer, and then those measurement values are memorized as a plurality of measurement data being related with those measurement values.
With this, it is possible to corresponds the fluctuation amounts on the front surface side and on the reverse surface side upon the basis the thickness of the sample piece to the detection values of said first and second detectors for the absolute measurement of distances.
When measuring the thickness of the wafer, the measurements are performed at the measurement points on the wafer which has the front surface and the reverse surface in height, lying within the height of the sample piece, on which such the correspondence is obtained, thereby obtaining the detection values of the first and second detectors. Searching conversely from those detection values by referring the correspondence data, it is possible to convert the respective detection values into the data of the relative fluctuations therefrom, upon the basis of the thickness of the sample piece. Since the thickness of the sample piece is constant, the fluctuations on both the front surface and the reverse surface must be substantially constant when being measured anyplace on the sample piece, however when a difference is generated in the fluctuation amount between on the front surface and on the reverse surface at the measurement point on the wafer, upon the conversion data thereof, that appears as an amount of difference with respect to the thickness of the sample piece.
In other words, the data being converted into the relative fluctuation amounts indicate the relative amounts of shifting in positions (i.e., the distances) upon the basis of the thickness of the sample piece, respectively. Then, by conducting addition and/or subtraction of the shifting amount on the front surface and the reverse surface from the thickness of the sample piece as a reference, it is possible to obtain the thickness of the wafer to be measured.
The value being actually calculated in this case is a sum of the relative measurement value and the known thickness of the sample piece, therefore the absolute values obtained as the detection values of the first and second detectors only function as a medium for obtaining the relative measurement values. As a result of this, at the time point when measuring the wafer thickness, the corresponding data are produced by the data sampling/memorizing means, therefore the accurate wafer thickness can be obtained by the relative measurement at every moment, without receiving ill influence of the temperature, and without a necessity of provision of an apparatus for the absolute measurement with high accuracy.